Atomic layer deposition (ALD) has gained acceptance as a technique for depositing thin and yet continuous layers of metals and dielectrics with high conformality. In ALD, a substrate is alternatively dosed with the precursor and one or more reactant gases so that reactions are limited to the wafer surface. Thus, gas phase reactions are avoided since the precursor and the reactant gases do not mix in the gas phase. Uniform adsorption of precursors on the wafer surface during ALD produces highly conformal layers at both microscopic feature length scales and macroscopic wafer length scales, and achieves a high density of nucleation sites. These attributes result in the deposition of spatially uniform, conformal, dense and continuous thin films.
The high quality films achievable by ALD have resulted in increased interest in ALD for the deposition of conformal barriers, high-k dielectrics, gate dielectrics, tunnel dielectrics and etch stop layers for semiconductor devices. ALD films are also thermally stable and very uniform which makes them attractive for optical applications. Another potential application for ALD is the deposition of AlOX as a gap layer for thin film heads, such as heads for recording densities of 50 Gb/in2 and beyond which require very thin and conformal gap layers. Conventionally sputtered gap layers are difficult to reliably scale below 300 Å due to excessive leakage currents. Although ion beam deposited gap layers can be scaled down in thickness to below 300 Å, such layers tend not to be adequately conformal. Further, process integration considerations for thin film heads of 200 Å or less tend to constrain the maximum deposition temperature to below 200° C.
Although ALD techniques support deposition of conformal thin layers, a number of difficulties exist which make commercial applications of ALD unworkable. One difficulty is that the repeated cycle of precursor and reactant adsorption and intervening chamber purges is time consuming, resulting in reduced throughput relative to conventional techniques. Another difficulty is that for a continuous monolayer of precursor to be adsorbed on the wafer without being thermally dissociated and for other process constraints, a relatively low deposition temperature is typically needed. However, the quality of the deposited thin film as measured by breakdown field strength tends to suffer at lower temperatures as impurities are incorporated in the film due to incomplete reaction of the precursor. Breakdown field strength comprises a good measure of film quality related to impurities, defects, roughness and film continuity.